The invention relates to an X-ray tube comprising an envelope containing a rotary anode which is rotatable about an axis. A motor rotor which is driven by a motor stator outside the envelope, drives the rotary anode. The anode is axially and radially supported by a bearing system.
Such an X-ray tube is particularly suitable for use as a radiation source in medical X-ray diagnostic apparatus.
U.S. Pat. No. 2,293,527 discloses an X-ray tube as described above. In this tube, the bearing system comprises two metal, lubricated ball bearings. To reduce wear in such an X-ray tube as much as possible, the anode is rotated substantially only when the X-ray tube emits radiation. The rotor is accelerated to normal operating speed before the tube is used, and the rotor is brought to rest after use. In spite of this, the life of the X-ray tube is short because the ball bearings have a short life.
The short life of each of the ball bearings is particularly due to the fact that a portion of the heat generated during operation of the anode is dissipated via the bearings. It is possible for the average temperature of the ball bearings to increase to approximately 300.degree. C. In addition, the ball bearings must operate in a vacuum. In such circumstances, ball bearings cannot be sufficiently lubricated so that the ball bearings must have a sufficiently large clearance to prevent the bearings from seizing up. Consequently, the X-ray tube is far from noise-free in operation. This noise is very irritating for a patient examined by an apparatus provided with such an X-ray tube. The life of each of the ball bearings is still further reduced by the fact that the tube current is passed through the bearings. This results in spark erosion of the bearing.